Avg Free is the most friendly anti-virus I have ever tried for an old slow PC.
Unlike the heavyweights, like Norton and McAfee, you will hardly know it is running. It will sit there in the background doing its job while allowing you to still do yours.
I have not had any problems with the background scanning locking up the PC or slowing it down to a crawl.
I have also not had any problems with my taskbar icons or opening folders in Explorer, if there are .rar files present or a shortcut to an application containing some type of server. I had both of these problems with some other anti-virus products I have tested, before.
I have been an AVG user for about 4 years. In all that time I have never had an incident where any version of AVG has crashed. I also have never been infected with any viruses or trojans. While an anti-virus does offer you protection, it is not a substitute for common sense. You still have to use your head and not do stupid things like opening strange email attachments from unknown sources.
So if you want protection against viruses and trojans, and want something that won't get in your way, AVG Free can't be beat. It's my #1 choice. And you sure can't beat the price (free).
Note: If you install AVG Free and later decide to uninstall it, you will have to use the original installer or download the latest version of AVG Fee and run the installer and use that to uninstall the older version. You can not uninstall it through add/remove programs.